In the highly competitive environment of emerging technologies, extension of wireless network radio signal coverage by use of various types of relays has recently been a target of intensive studies. An amplify-and-forward relay amplifies a received analog signal, and transmits the amplified signal forward. This type of amplification improves the signal power but tends to amplify also the noise received with the actual content of the signal. A decode-and-forward relay regenerates the signal and transmits the regenerated signal forward. During the regeneration, the decode-and-forward relay may apply various digital operations on a received signal. Due to transmission control measures during the regeneration stage, a decode-and-forward relay typically provides better signal quality than an amplify-and-forward relay. Use of decode-and-forward relays to extend the coverage of a single base station is considered to improve the capacity usage of the single base station and to enhance the signal quality received by user equipment.
In communication systems, a protocol defines procedures that provide communication between two or more functions within the same layer in a hierarchy of functions. A generic concern in introducing decode-and-forward relays, or any type of operative substations that perform functions on the received signals in the interface between base station and user equipment, is how to divide information and responsibilities between the operative substation and the base station in such a way that functions in any protocol layer may be implemented without problems or complications, for example in the form of increased delays or signaling load. As an example, in order to be able to provide a base station type radio interface to user equipment direction, a relay node needs to be able to provide several lower layer base station functions for radio resource management, radio bearer control, radio admission control, connection mobility control, and dynamic allocation of resources to user equipment in both uplink and downlink. On the other hand, the base station needs to be aware of the status and related information on at least part of these functions in order to appropriately perform the upper layer functions relying on this information.
A further issue related to introduction of relays is device compatibility. It is likely that relay extensions will be mainly introduced to existing networks with a considerable installed base of network nodes and user equipment. Careful management of compatibility issues is very important, because the infinite number of users, user equipment versions, and user equipment manufacturers eliminate any large-scale transitions to new protocols or procedures. It is therefore essential that user equipment do not need to recognize whether the signal it receives comes from a base station or from a relay node. This means that the transmissions by the relay node should be performed using the same protocols and procedures as in the original transmission by the base station.
A conventional approach to tackle such challenges is to make the intermediate node, here the relay node, to mediate any necessary information in both directions. However, in advanced radio networks, many of the radio interface operations are intensively interactive and would significantly suffer from delays evidently resulting from such message mediation. The increased exchange of signaling messages would also unnecessarily load the overall system resources.